Field of the Disclosure
The present application relates to a liquid crystal display device, and more particularly, to a backlight unit adapted to minimize or prevent a light leakage fault generated along the edge of a liquid crystal display panel.
Description of the Related Art
Liquid crystal display (LCD) devices belong to technology-intensive and high value-added display devices. Such LCD devices exhibit low power consumption and superior portability.
An active matrix type LCD device includes thin film transistors which are used as switching elements and perform voltage-on/off control for each pixel. Such an active matrix type LCD device has high definition resolution and superior motion picture realization capability. In general, the LCD device includes an LCD panel which is fabricated by preparing two substrates, such as a so-called array substrate and a so-called color filter substrate and interposing a liquid crystal material between the two substrates. The array substrate is fabricated through an array substrate fabrication procedure which forms thin film transistors and pixel electrodes on a substrate. The color filter substrate is fabricated through a color filter substrate fabrication procedure which forms color filters and a common electrode on another substrate. The interposition of the liquid crystal material is performed by using a so-called cell process.
The LCD panel is not a self-emissive panel, but rather, the light transmittance of the LCD panel is controlled to display images. As such, a separated light source is necessary for the LCD panel. In accordance therewith, the LCD device includes a so-called backlight unit which includes a light source and is disposed near or under the rear surface of the LCD panel.
The backlight unit can be classified into a direct type and an edge type on the basis of the position of the light source. The direct type backlight unit allows the light source to be disposed under the LCD panel. As such, the direct type backlight unit emits light from the light source to be directly applied to the LCD panel. Meanwhile, the edge type backlight unit includes a light guide plate disposed under the LCD panel and a light source disposed at least one side surface or edge of the light guide plate. In such an edge type backlight unit, light emitted from the light source is indirectly applied to the LCD panel as a result of the total internal reflection through the light guide plate.
As a light source of the backlight unit, fluorescent lamps including a cold cathode fluorescent lamp (CCFL) and an external electrode fluorescent lamp (EEFL) have been mainly used. Nowadays, as the LCD device become thinner and lighter, the fluorescent lamps are being replaced by light emitting diodes (LEDs) which have advantages in power consumption, weight and brightness.
FIG. 1 is a planar view showing an LCD device of the related art in which a bluish light leakage fault is generated. FIG. 2A is a cross-sectional view showing the structure of a backlight unit disposed in an LCD device of the related art. FIG. 2B is a graph illustrating wavelength characteristics of lights output from components of the related art backlight unit shown in FIG. 2A.
Referring to FIGS. 1, 2A and 2B, the LCD device 10 of the related art includes an LCD panel 30 and a backlight unit 50, which are combined with each other by an upper cover 20 and a lower cover. The upper cover 20 can be formed in a tetragonal rim structure with an opened central portion, but may have various other configurations. Such an upper cover 20 supports the edge of the LCD panel 30 surrounding a display area.
The backlight unit 50 includes a light source 60 having an LED package 60b which includes at least one LED 60a, and a light guide plate 51 configured to distribute the light generated by the light source 60 such that the overall surface of the light guide plate is configured to illuminate. Also, the backlight unit 50 includes a light conversion sheet 52, a first optical sheet 53 and a second optical sheet 54 which are sequentially disposed on the light guide plate 51.
The light conversion sheet 52 can be formed from a resin which has a refractive index and includes red quantum dots or green quantum dots, when the light source 50 is a blue light source. The quantum dots can also be referred to as quantum rods, nano-crystal, or other types of materials having similar quantum mechanical characteristics. As such, the light conversion sheet 60 can realize white light with a wider color gamut. White light with the wide color gamut means that each spectrum of the primary colors have a narrow FWHM (full width at half maximum) value as shown as a dashed line 54 in FIG. 2B.
In general, the light source 60 of the backlight unit 50 can emit blue light. In this case, the light guide plate 51 can output only light of blue wavelength. However, the light conversion sheet 52 can output red, green and blue lights generated by the quantum dots on the basis of the blue wavelength input from the light guide plate 51.
In other words, as shown in FIG. 2B, blue light input to the light conversion sheet 52 is converted into red and green lights by the scattered quantum dots within the light conversion sheet 52. As such, the light conversion sheet 52 can output red, green and blue lights toward the optical sheets.
However, only a portion of blue light may be converted into red and green lights and the remaining portion of blue light passes through the light conversion sheet 52 without any light conversion. Due to this, light output from the light conversion sheet 52 may become bluish light.
The first and second optical sheets 53 and 54 are configured to reflect a portion of light output from the light conversion sheet 52. A portion of reflected blue light can be additionally converted into red and green lights by the scattered (or distributed) quantum dots within the light conversion sheet 52. In accordance therewith, light output from the optical sheet can be converted into white light.
However, without the compensation pattern of the present disclosure, the frequency of hitting quantum dots at the edges of the light guide plate is not sufficient. That is, more blue light is output at the edges of the light guide plate compared to other areas of the light guide plate. With the compensation pattern, the light conversion sheet can provide more chances of converting the blue light into the green light and red light such that the bluish color displayed portions on the screen can be reduced as a result of overall white balance being achieved. In other words, a higher probability of hitting the quantum dots with the blue light can reduce undesirable blue light outputs, which increases the amount of the red and green light outputs to thus achieve better white balance across the entirety of the display screen. Therefore, undesirable bluish light can be converted into white light that is output more uniformly via the light guide plate.
That is, more chances to hit the quantum dots with the blue light can reduce the portion of the blue light output, but increase the output amount of the red and green lights. Therefore, bluish light can be adjusted to be converted into white light.
In this manner, the color conversion degree of blue light can be controlled by the configuration of optical sheets and the amount of the scattered quantum dots within the light conversion sheet 52.
However, as seen from the related art LCD device 10 of FIG. 1, bluish light BL is leaked along edges of the LCD panel 30. This results from the fact that more blue light is outputted at the edges of the light guide plate 51 compared to the other regions of the light guide plate 51. In other words, because the quantity of blue light output from the edges of the light guide plate 51 is larger than color conversion capacity of the light conversion sheet 52 and the optical sheets, light output from the edges of the LCD panel 30 can be perceived as a bluish color.
Light output through the first optical sheet 53 and the second optical sheet 54 can adjust its white balance at majority area of the related art LCD device 10 of FIG. 1. However, the light output at the edges of the light conversion sheet 52 and the light guide plate 51 output bluish light or blue light. Due to this, bluish light leaked at the edges of the LCD panel 30 deteriorates the image quality of the related art LCD device 10 FIG. 1.